1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a light emitting diode (LED) printer head as the exposure section thereof.
2. Description of Related Art
In recent years, an image forming apparatus using an LED printer head (hereinafter referred to as an LPH) as the exposure section thereof to form an electrostatic latent image on the surface of the photosensitive body thereof has been developed. In the LPH, LED chips, each of which includes a plurality of LED elements arranged in the main scanning direction according to a preset resolution, are arranged in an array, and the LPH includes an optical section, such as a graded index (GRIN) lens, which condenses irradiated lights emitted from the LED elements according to image data to form the electrostatic latent image on the photosensitive body.
It is known that the unevenness of light quantities is generated in such an LPH owing to the dispersion of the LED elements in manufacturing, the optical properties of the GRIN lens, and the like. The following technique in order to settle the unevenness of the light quantities is known. That is, the technique previously stores light quantity correction data into a nonvolatile storage section, from and to which data can electrically be erased and written, such as an electronically erasable and programmable read only memory (EEPROM). The light quantity correction data is for evening out the light quantities of a plurality of LED elements by digitally controlling the current values of driver circuits to light the LED elements. Moreover, the technique reads the light quantity correction data stored in the storage section into the control device thereof to wholly control the image forming apparatus to perform an exposure by the use of image data and the read light quantity correction data.
Moreover, the resolution in the main scanning direction (arranging direction of the LED chips) has been becoming higher to be 600 dpi, 1200 dpi, and so forth, with the realization of the densification of the arrangements of the LED elements. For example, if the maximum size of a recording medium on which an image forming apparatus can form an image is the A3 wide size (324 mm in width direction), then 7680 elements and 15360 elements of LED elements are arranged in the cases of 600 dpi and 1200 dpi of resolution, respectively.
In this manner, as the number of LED elements has increased with the heightening of the resolution, the data quantity to be controlled as image data has increased. Moreover, the exposure control method has also changed from only the one to perform simple on-off actions of the LED elements to the ones including the one to control a lighting exposure time on the basis of set numerical values of a plurality of bits, and consequently the traffic of the data to be used for an exposure has also increased. Consequently, it has become indispensable to mount an LPH interface equipped with a large-capacity and high-speed data communication function in order to meet the demand of the improvement of the productivity (high-speed capability) of the image forming apparatus.
Furthermore, to make it possible to form images on various recording media, an electrophotographic image forming apparatus is equipped with a plurality of image formation speeds in the same image forming apparatus according to the features (such as paper types and paper thicknesses) of various recording media in order to improve the fixation performance of toner, and the electrophotographic image forming apparatus must meet the data communication functions capable of coping with the image formation speeds according to the various recording media.
As an interface technique to realize the large-capacity and high-speed data communication function to the problems mentioned above, for example, there is a technique to attain the high-speed transfer of multi-bit data by the following measures. That is, on the transmission side, the technique performs the parallel-serial conversion of clock-synchronized parallel data by a low voltage differential signaling (LVDS) circuit, and performs the clock modulation according to the number of bits of the serial conversion by a phase locked loop (PLL) circuit. On the reception side, the technique restores the converted serial data to the input parallel data by performing the serial-parallel conversion of the serial data and restoring the modulation clock to the original clock by a receiver circuit equipped with a frequency modulation circuit. By that way, the technique attains the high-speed transfer of multi-bit data.
By adopting the technique mentioned above, it is possible to realize the large-capacity and high-speed data communication function by arranging a control signal, image data and light quantity correction data in parallel data, and the degree of freedom of the length of a bundled wire becomes high to make it possible to heighten the degree of freedom to the layout of the inside of the image forming apparatus. Accordingly, it becomes possible to intensively arrange the high-speed data processing section to unitize it.
However, although the technique mentioned above enables the high-speed communication of exposure data and the like to the LPH, the method increases the costs of circuit components and increases the production cost with the increase of the circuit components when the method is adopted as the means for reading the light quantity correction data of the LPH, and consequently the method causes a user an disadvantage. Moreover, the method has a problem of the impossibility of making the most of the performance of the LVDS circuit for exposure data owing to the limitation of the length of the bundled wires caused by the restriction of the circuit configuration to read light quantity correction data.
There is a method of mounting a storage section (for example, ROM) storing light quantity correction data inside the image forming apparatus as the means for solving the problem. However, because an image forming apparatus that is required to have a high speed and high durability needs the exchange of an LPH at the time of maintenance and the adjustment of light quantity correction data according to the process conditions and the frequency of usage, it is necessary to update the data of the storage section or to exchange the storage section every operation of the maintenance and the adjustment. Consequently, if the light quantity correction data suited to the LPH is not stored in the storage section owing to a trifling operation error, the incongruence becomes a cause of producing an image defection. Furthermore, the management of light quantity correction data is necessary also in the manufacturing process of the image forming apparatus, and the collation of the LPH mounted in the image forming apparatus with the light quantity correction data stored in the storage section becomes necessary to produce a new technical problem.
Consequently, a technique to provide a nonvolatile storage section that stores light quantity correction data in the LPH, and to read the light quantity correction data from the storage section has become general.
For example, Japanese Patent Application Laid-Open Publication No. 2001-239697 discloses an apparatus to control the lighting of LED elements by reading light quantity correction data from an EEPROM (storage section storing correction data) by a strobe signal, by supplying the read light quantity correction data to an LED driver IC as printing data, and by supplying a selection signal of LEDs with a strobe signal as an drive instruction of an LED array according to the printing data.
The Japanese Patent Application Laid-Open Publication No. 2001-239697 discloses that the technique makes a drive section (printing control section) generate a clock signal to obtain the light quantity correction data, and that the technique inputs the light quantity correction data stored in the storage section into the drive section in synchronization with the generated clock to transfer the light quantity correction data by the supplied clock. The technique aims to reduces the design margin accompanying a timing changes in the transfer clock of the printing data and the transfer clock of the light quantity correction data, that is, the technique separately considers the reading control of the light quantity correction data, the setting control of the light quantity correction data, and the transmitting method of the printing data, and the technique provides an interface circuit to balance each of them so that they can be processed severally by suitable methods.
However, because it is necessary for the conventional technique mentioned above to balance the interface of the whole image forming apparatus in order to improve the throughput of large-capacity light quantity correction data and the data communication capacity thereof (high-speed capability and reliability of transmission signal), the printing ability thereof is sacrificed. Consequently, the communication capacity of exposure data is limited by the reading and setting functions of the light quantity correction data from the storage section. Moreover, in the case where the length of the bundled wire is desired to be elongated, the length is also restricted by the interface circuit.